They both rotate with the same speed, but are they fully aligned? [In the pair of sample pentagons on the right, only the left pair is ‘aligned’.]

With the “compensate” slider (or the cursor keys ← and→) you can adjust the relative alignment. Below the slider, the alignment angle is given, for me around –2.5° looks aligned.

Once you are satisfied they match, press ‘Stop’, or tick the ‘light background’ checkbox.

Do you see a misalignment now?

Apparent alignment depends on rotation speed. It also depends on the luminance difference, the darker the outer “shadow”, the stronger the effect.

You can try out various speeds and various shapes. If you see a counter-rotation when stopping the shapes: that’s the motion after-effect. It is also possible to choose a darker blue using the colour pickers at the left. With the RGB setting ‘#000033’ the effect is nearly double in size.

Comment

The main mechanism here is that processing speed within our visual system depends on luminance, “the brighter the faster”. A rough rule of thumb is: 10× brighter → 10 ms faster.

The mechanism is located within the photoreceptors in the retina. One might think that the optional bright-background effect argues against this, because the dark shape remains dark – but there its edges are defined by the bright background.

The present demonstration could be seen as a rotatory version of the “Hess effect”, where the more intensely illuminated of a pair of laterally-moving targets appears displaced ahead of the other. However, possibly due to pursuit eye movements, this is more difficult to demonstrate. There is also a close relation to the Pulfrich phenomenon, which, however, can be seen in binocular vision only.

References

This display was suggested to me by Lars Frisén in August 2006 and I heartily thank him for his ideas and graciously allowing the result to be published here; he also invented the name “Lazy Shadow”. He now has set up his own page on this topic.